On Tue, May 30, 2023 at 3:30 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, 30 May 2023 15:24:40 -0700 Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > > Before storing a page, zswap first checks if the number of stored pages > > exceeds the limit specified by memory.zswap.max, for each cgroup in the > > hierarchy. If this limit is reached or exceeded, then zswap shrinking is > > triggered and short-circuits the store attempt. > > > > However, since the zswap's LRU is not memcg-aware, this can create the > > following pathological behavior: the cgroup whose zswap limit is > > reached will evict pages from other cgroups continually, without > > lowering its own zswap usage. This means the shrinking will continue > > until the need for swap ceases or the pool becomes empty. > > > > As a result of this, we observe a disproportionate amount of zswap > > writeback and a perpetually small zswap pool in our experiments, even > > though the pool limit is never hit. > > That sounds unpleasant. Do you think the patch should be backported > into earlier (-stable) kernels? I think it should be, for any kernel version after f4840ccfca25. > > > This patch fixes the issue by rejecting zswap store attempt without > > shrinking the pool when obj_cgroup_may_zswap() returns false. >
